Glycogenolysis Flashcards
glycogenolysis
The breakdown of glycogen to get free glucose. Occurs in the liver and muscle.
glycogen phosphorylase
The enzyme that degrades glycogen from the nonreducing ends of the glycogen molecule. Glycogen phosphorylase catalyzes a reaction that yields glucose 1-phosphate and the cleavage of the glycosidic bond provides the energy to add the phosphate (i.e. no ATP is needed). It can clip alpha 1-4 bonds up until 4 sugars away from a branch point.
phosphoglucomutase
converts glucose 1-phosphate into glucose 6-phosphate. The mutate puts phosphate onto carbon-6, THEN removes the phosphate on carbon 1. This swap doesn’t require any energy.
4 enzymes needed to release glucose from glycogen
- glycogen phosphorylase
- transferase
- alpha 1-6 glucosidase
- glucomutase
In liver: 5. Glucose 6-phosphatase
Transferase
shifts the 3 sugars near a branch point down onto a longer chain so the glycogen phosphorylase can keep chewing them up. It leaves one sugar attached at the branch point for another enzyme to cleave.
alpha 1-6 glucosidase
cleaves the single sugar off the branch point (cleaves the alpha 1-6 glycosidic bond). This is the deb ranching enzyme
Muscle
Once glycogen has been degraded to glucose 6-phosphate, the glucose-6-Pi can be used in glycolysis to provide energy for muscle. Muscle stockpiles glycogen for its own use.
Liver
Once glycogen has been degraded to glucose 6-phosphate, an enzyme in the liver - glucose 6-phosphatase - removes the phosphate so that glucose can be released into the blood and travel to other tissues. Liver stockpiles glycogen so it can regulate blood glucose levels.
Insulin
favors glycogen synthesis. Insulin is released when blood glucose levels are high in the blood. Insulin is a protein hormone.
Glucagon
favors glycogenolysis. Glucagon is released when blood glucose concentration is low and signals glycogen to be broken down so glucose can be released. Glucagon is a protein hormone.
epinephrine
favors glycogenolysis. epinephrine tells the muscle to get glucose out of its glycogen in muscle. Epinephrine is a small molecule made from modified amino acids.
Phosphorylase A
the active form of phosphorylase that breaks down glycogen. Phosphorylase A contains two phosphate groups. Phosphorylase a favors the R state.
Phosphorylase b
the inactive form of phosphorylase. Has no phosphates. Favors the T state.
How liver regulates phosphorylase
Liver phosphorylase is normally in the active state (a form in the R state). In liver, phosphorylase is always prepared to generate glucose for the rest of the body unless signaled otherwise by high glucose levels in the blood. High glucose stabilizes the T state of the liver phosphorylase. When glucose binds, it shifts equilibrium toward T state, but it doesn’t affect the phosphates bound to phosphorylase a.
How muscle regulates phosphorylase
Muscle phosphorylase is inactive (the b form in the T state) unless it’s turned on. Muscle phosphorylase is activated by a high concentration of AMP (low energy charge). AMP shifts equilibrium of T states to more R states.
